As consumer electronics products are more and more powered by rechargeable batteries it is important to provide means for recharging the batteries. Rechargeable batteries offer lower lifetime cost to the consumer. Also, rechargeable batteries may allow the design of a product enclosure that does not require means for easy access to a battery for replacement. Batteries which do not require replacement may also allow the manufacturer to utilize a custom, nonstandard battery form factor which in turn may allow for a smaller or more ergonomic end product.
Rechargeable batteries typically involve a chemical process which delivers current when the positive and negative terminals are connected across a load, the process being reversible (charging) by the application of a voltage sufficient to cause a net current to flow into the battery. The charging process, then, provides electrical energy which is stored and later may be released. The predominant chemistries used today are those using some form of lithium, nickel, cadmium, or lead, though many other chemistries are also used.
Batteries based upon a lithium-ion (Li-ion) chemical reaction are used in many products. Li-ion battery manufacturers specify the charging method and various parameters for charging a battery. The predominant method specified is for a charger to charge a Li-ion battery using a constant current until a certain voltage is attained (for example, 4.2 volts), then to provide a constant voltage for an additional period of time until the charging current goes down to a certain level, which is defined as the end-point condition. This method is denominated the “CC/CV” or “Constant Current/Constant Voltage” method. In a standard CC/CV charging profile, measurements of battery voltage are only used to determine when to switch from constant current charging to constant voltage charging, after which charging current is monitored for end-point determination.
Multiple charge/discharge cycles slowly erode the battery electrodes, decreasing a battery's electrode surface area. Thus, as a battery ages, the current density resulting from charging at a specified constant current increases, more rapidly degrading the performance and lifetime of the battery. There is variation from battery to battery of a common type due to manufacturing variations. Even a given battery will have more power available (and be capable of accepting a more aggressive charging profile) when fresh, undamaged, and warm. Likewise the same given battery will have less power available (and require a correspondingly less aggressive charging profile) after many charge/discharge cycles, when damaged, or when cold. Thus the CC/CV charging method may undercharge or take longer than necessary for some specific battery units, or may overcharge or charge too rapidly for other units, thus causing them damage which shortens battery lifetime as well as giving the user a less satisfactory experience. The industry, then, has need for a charging solution that adjusts to the actual instant condition of a battery such that any given charging cycle is as short as possible but without damage and avoiding undue deterioration of the performance of the battery.